What Is a Blackbody?

I search for a definition of the concept of blackbody in two new posts on The World as Computation:

• What Is a Blackbody, Really?
• Universality of Blackbody Radiation
• Grey vs Black Body.

I come to the conclusion that a blackbody is a chosen reference body with maximal cut-off frequency. Universality of blackbody radiation then reflects choosing a specific body as universal reference rather than identifying a universal property shared by all bodies.